1. Field of the Invention
The present invention relates to an apparatus and method for calculating an environmental condition, such as altitude, from the sensed values of other environmental conditions and the flying height of a head in a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads can magnetize and sense the magnetic fields of the disks to write and read data, respectively. The heads are coupled to a pivoting actuator arm that has a voice coil motor. The disks are rotated by a spindle motor of the drive.
Data is typically stored on tracks that extend radially across the disk surfaces. The voice coil motor can be energized to pivot the actuator arm and move the heads to different track locations. Each track is typically divided into a number of sectors. Each sector contains at least one data field. Data is typically written onto a number of sectors during a write operation of the drive.
Each head has an air bearing surface. The rotating disks generate a flow of air that cooperates with the air bearing surface to create an air bearing between a head and the adjacent disk surface. The air bearing eliminates or minimizes mechanical contact between the head and the disk.
The disks are magnetized and data is written with a magnetic field that emanates from the heads. The magnetic field is created by providing a current to a coil that is embedded into the head. Magnetization of the disks is a function of the height of the air bearing. A taller air bearing will produce a weaker magnetic field and vice versa.
The height of each air bearing is a function of the temperature, humidity and altitude within the disk drive. For example, the air bearing will become smaller with a increase in temperature and a decrease in altitude. The flying height of the head will decrease with an increase in humidity.
There are a number of parameters associated with writing data onto a disk. By way of example, there are write current, overshoot control, and pre-compensation parameters associated with each write channel of the drive. These parameters are typically stored in the disk drive and retrieved each time the drive is booted up.
Ideally, to compensate for varying temperatures and altitudes the disk drives are calibrated in a temperature and altitude simulation chamber to determine the optimum write parameters for different temperature and atmospheric conditions. The different write parameters can then be stored in a table before shipping the disk drive from a manufacturing facility. When the drive boots up, the temperature and altitude are sensed and the corresponding write parameters are pulled from memory.
Unfortunately, it is not economically feasible to test every single disk drive in temperature and altitude simulation chambers in a manufacturing facility. The equipment and energy cost would significantly add to the final cost of each disk drive. In addition, such test chambers would be difficult to manage on a large scale since such equipment is complex and breaks down easily. Currently, all drives are tested at or near sea level at room temperature.
Another draw back to prior temperature/altitude compensation schemes is that altitude sensors are relatively expensive and increase the overall cost of the drive. It would desirable to compensate for changes in altitude without requiring an altitude sensor.